Ctrl+V in TUI is image-only when it reaches Codex as a key event; add symmetric text paste

## Summary

In the TUI, Ctrl+V currently behaves as an image-paste shortcut if it reaches Codex as a key event.

That means when the clipboard contains text, and the terminal does not convert the paste into a bracketed Paste(...) event, Codex tries to
paste an image instead and shows an error like:

> Failed to paste image: no image on clipboard: ...

## Root cause

There are currently two different paste paths:

• text paste: relies on terminal/crossterm Event::Paste(...)
• Ctrl+V: handled as clipboard image paste in the TUI keybinding layer

So if the terminal forwards Ctrl+V as a keypress instead of surfacing a paste event, text paste does not happen inside Codex.

## Proposed behavior

• Ctrl+V in local desktop sessions:
• try clipboard text first
• if non-empty text exists, route it through the existing composer paste path
• if no text exists, fall back to image paste
• Alt+V:
• remain explicit image paste
• SSH/remote sessions:
• do not do process-local text clipboard reads
• rely on terminal-owned paste
• if Ctrl+V reaches Codex as a key event, show a short hint instead of reading the remote process clipboard

## Why this shape

• preserves terminal-native Paste(...) handling
• preserves explicit image paste
• avoids incorrect clipboard semantics in SSH sessions
• makes Ctrl+V symmetric for local users whose terminal forwards the keypress

## Local implementation + verification

I implemented this in a local clone and verified with:

• just fmt
• cargo clippy -p codex-tui --tests
• cargo test -p codex-tui

The change includes:

• text clipboard read helper
• WSL text fallback
• Ctrl+V text-first shortcut policy
• Alt+V image-only policy
• SSH hint path
• updated footer/snapshots/tests

If this approach matches the intended direction, I can turn it into a PR if invited.